1. Field of the Invention
The present invention relates to an image display device and method for displaying an image signal selected from a plurality of image signals input from the outside and switching the image signal being displayed as desired.
2. Description of the Related Art
In the art of image display devices, it is known to select one of a plurality input image signals using a mechanical or electrical switch and display the selected image signal.
FIG. 14 is a block diagram illustrating an example of such a conventional image display device.
In FIG. 14, a plurality of personal computers (PCs) 101, a DVD (digital versatile disc) player 102, and a terminal 103 for outputting an image signal are connected to a display device 105 via an image signal selection switch (SW) 104. The display device 105 displays an image signal selected by the image signal selection switch 104, that is, an image output from one of the personal computers 101 or the DVD player 102.
When the convention image display device is used, in a meeting or the like, to display images such that an image signal is selected from those output from a plurality of personal computers or terminal devices connected to the display device and the selected image signal is displayed using a large-screen display device such as a projector, there is a possibility that the image signals output from the PCs or the terminal devices are based on different standards, and thus it is required that the image display device should have the capability of handling the image signals based on various different standards. Even if the image display device has such a capability, the image display device can still have a problem. That is, when the input image signal is switched, the image mode is detected from the synchronization signals HD (horizontal deflection) and VD (vertical deflection) after the new input signal has become stable, and then the oscillation frequency of a PLL circuit is determined and the displaying position and the sampling phase are adjusted. Therefore, it takes a long time for the displayed image to become stable. The displayed image can have a disturbance during such a transient period.
Another problem of the conventional image display device is in that only one image signal selected from the plurality of input image signals can be displayed at a time, and thus it is impossible to know the contents of the other image signals.
When a projector is used in a meeting to display an image signal selected from image signals output from a plurality of PCs and the image being displayed is switched as required, it is impossible to know the contents of the image signals other than the image signal being currently displayed. This is inconvenient for the attendants at the meeting. The image signals output from the respective PCs can be different in characteristics (such as a frequency) from each other, it takes a long time to achieve a stable state after switching the image being displayed even if the display device has the multiscan capability, and the image being displayed can be disturbed. In particular, in a meeting or education system in which a large-screen display device is used to display an image signal selected from the image signals output from a plurality of PCs and switch the image being displayed as desired, the above-described problems are serious.
In recent years, the quick and great advance in the technology of mobile terminal devices have made it possible for ones to store necessary data in their own mobile terminal devices and to obtain data by accessing a server via their own terminal devices. This allows ones to do jobs without having restrictions in terms of time and place, and thus greater productivity of jobs can be achieved. When a meeting is held in such an environment, attendants want to make a discussion on the basis of data output from their own mobile terminal devices.
However, in the conventional image display device, the simple selection switch 104 used to connect a plurality of devices causes the following problems. That is:
1) Complicated and troublesome cable connections are required. In particular, the small size of mobile terminal devices results in a limitation in the type of cable. Furthermore, in many cases, an additional module is required for connection to the video output terminal. This makes the cable connection more complicated and troublesome. A connection to a projector for displaying on a large screen an image output from a PC in a meeting is generally made using a VGA (Video Graphic Array) cable having a D-Sub (Subminature-D) 15-pin connector with a large size. The large connector size and the recent tendency of projectors to have reduced sizes limit the number of input terminals of the projectors up to two, one of which is generally used for connection with a PC and the other is used as a video input terminal. Although some high-end projectors have two or three input terminals for connection with PCs and for video inputs, respectively, the number of devices which can be connected is still limited.
2) When a VGA cable is used, the VGA cable having a large diameter and a small length on the order of 2 m limits the location of a mobile terminal device. Thus, an attendant has to move his/her mobile terminal device to a location which allows the terminal device to be connected to the cable whenever it is required to output data in a meeting. Thus, time is spent uselessly for connection, and the connecting operation often breaks the meeting.
3) It takes a long time to achieve a stable state in the displaying of an image after switching the image being displayed. This has been described in detail above, and thus no further description is given.
4) only the image signal output from the device of the presenter is displayed, and the images of the other attendants cannot be displayed. This has also been described above, and no further description is given.
5) It is impossible to create or edit data from a plurality of data and transmit the resultant data to the attendants.
When a discussion is made among a large number of attendants, it is desirable, in order to achieve high productivity in jobs, not only to present data of the respective attendants but also to distribute the conclusion of the discussion to the attendants immediately in the meeting. However, the conventional image display device is only capable of displaying one received image signal, and is not capable of distributing the data being displayed. Thus, needs described above cannot be met.